Vacuum tube



H. A. SNOW VACUUM TUBE July 21, 1936.

Original Filed Dec.

fay rozrs ATTORNEY Patented July 21, 1936 UNITED STATES PATENT OFFICE VACUUM TUBE Harold A. Snow, Mountain Lakes, N. J., assignor to Radio Corporation of America, a corporation of Delaware 11 Claims. 250-2'7.5)

The present application relates to space discharge tubes of the type including a cathode, two grids and an anode, and is a division of my application, Serial No. 437,225, filed December 13, 1930 which in general relates to variable mu tubes.

The method of operation and the advantages of vacuum tubes of the variable mu type disclosed by this application are discussed more fully in the aforesaid application and in the Proceedings of the Institute of Radio Engineers for December, 1930, wherein in an article by Stuart Ballentine and Harold A. Snow, entitled Reduction of distortion and cross-talk in radio receivers by means of variable-mu tetrodes there is demonstrated the intimate relationship between the problems of distortion and cross-talk in radio frequency amplifiers, and their elimination by means of variable mu tubes;

It is, therefore, one of the main objects of the present invention to provide a space discharge tube having such operating characteristicsthat no distortion is introduced when, for increasing signal strengths, the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of the maximum amplification.

A further object of the invention is to provide an electron discharge tube having such characteristics, that when the potentials are adjusted to give a relatively low amplification of strong signals, the'change in transconductance for a given change in the gain control voltage is much lower than is the case with the known types of tubes.

Another, and important object of the invention is to provide a high frequency amplifier tube of the type which has two grids and'which is capable of adjustment to give an undistorted output of approximately constant magnitude over a wide range of applied carrier voltages.

A more specific object of the invention is to provide an electronic amplifier of the screen grid type in which different portions of the electron stream are influenced at different rates by the voltages applied to the control grid.

The novel features which I believe to be characteristic of my invention are set forth'in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several arrangements whereby my invention may be carried into effect.

In the drawing, .Fig. l is a perspective view, partly in section, of a screen grid tube embodying the invention,

Fig. 2 isa diagrammatic view illustrating another embodiment of the invention, and if Fig. 3 isa curve sheet showing the variations of plate current with grid bias for a tube such as shown in Fig. 1.. I

Referring to the accompanying drawing wherein like reference characters in the diiferentfigures designate the same elements, the tube shown in Fig. l is a form of. tube known commercially as a screen grid tube, with anequipotential cathode having a separate heater for the cathode. 15 As is Well known, this particular type ,of tube vcomprises an evacuated envelope enclosing a cathode C, heated by a resistance heater (not shown) within the cathode, an. inner grid CG, a second or screen grid SG,-a plate, or..anode P, and an outer screen S which is electrically connected to the second grid. Except for the novel construction of the control grid CG and of the second or screen grid, the several ,ele-

the order of twicethe'pitch of the winding. The

windings of each section are of the same pitch, which may be the same as that now employed in tubes of this type. This particular physical embodiment of the invention therefore physical- ,ly differs from the known construction, of the same general physical design, by the absence 'of two complete circumferential turns of the control grid winding. V t

Thisparticular construction results in a tube in which the control exercised upon the electron stream is not uniform over the entire extent thereof. 1

The operation of the tube shown in Fig. 1 may be explained as follows:

The control grid has been shown to be divided into two sections which are mounted witha; gap between them. At low negative biases the entire cathode is operative, and the tube has aboutthe same characteristics it would have if the gap V were in place. As the grid bias increases negatively the electroncurrent through the upper and lower parts of the control grid are cut off leaving a low mu control through the gap. At these bias voltages the tube acts as if the upper and lower sections, of the control grid were formed of solid metal, and controlled the current through the gap in the ordinary manner.

It will be apparent that various combinations of the described constructions may be included in a single tube. As shown in Fig. 2, the screen'grid 3f;

SG may comprise twocylindrical sections of dif-' ferent diametersand the control grid CG may be one wherein the winding pitch isnot uniform throughout the entire length of the grid; the grid shown in Fig. 2 .being'of progressively varying pitch from one end to theother....

To preventthe flow of an excessive plate c'urrent the larger diameter section of thefscreen grid is located opposite the portion of the grid of greater pitch. 7 j

The mode of operation of the modification disclosed in Fig. 'Zcan be explained in the same manner as has been explainedin connection with the embodiment shown in Figgl. Essentially, and basically considered, all of theitube structures disclosed herein provide a radio frequency slope of the curve'atany point is a measure of the tion'at that point, i 'In'Fig. 3,"thesolid line curve'A is the transfer characteristic for a tube such as shown in Fig. 1, and the dotted line curve Bis a similar curve for a commercial screen grid tube of the same genam'plification when the tube is biased. for operaeral typebut' having a continuous control grid winding of uniform pitch. An examination of curve A shows that, with tubes embodying the invention, a control of amplification extends over a range .of controlgrid bias offromlzero to more thanf-30 volts. With the'known tubes, the curvature of the-transfer characteristic-approaches .zero'at av control grid bias ofabout -15 volts. In other words, an increase of the grid bias above approximately15 volts negative will not i be' accompanied by' a decreasein amplification when the known type of tube construction is employed, but with tubes embodying'the invention, the amplification may be: varied with changes of control grid bias throughout a range of from zero toupwardly of -30"volts.; The tubes will ,still pass.signals,'by leakage transmission when the control grid biases exceed these respective values, but control of amplification is no longer control grid, and a .said screen grid.

curves of Fig. 3 will show that the maximum curvatureof curve A is substantially that of curve B.

. It is to be unerstood that the physical construction of the control grid, or the geometric and structural relationships of the tube elements are lower. than subject to wide variation so long. as the control xamplifientubes operating in parallel, one tube having a relatively high ratio of plate voltage to control grid voltage (high mu), and one or more, of the remainingv tubes having lower ratios of usually be more economical and convenient than .the parallel tube arrangement- By transconductance is meant the ratio; of

electrode to the change inthe voltage onj another electrode, under the condition that all otherivoltiages remain unchanged. By mu-factor is meant the ratio of the change in one electrode'voltage to a change in the other electrode voltage, uh-

der the condition that a specified current remains unchanged. V

ile I have indicated anddescribed a pre- 'ferred way: of carrying'my invention into effect, it will be apparent to one skilled in the 'art that my invention is byino means limited to 'thep'articular organization 'shown and described, but

that many modificationsmay be made without departing from the scope of my invention assetforth in the appended claims.

'- WhatIclaimis:

15 plate voltage .to controlgrid voltage; (low mu); -,A singletlibe exhibiting these characteristics will .20 the change in the current in the circuit-of an '1. 'A vacuum tube'comprising'a"cathodepja screen grid and a plate, control means of progressively varying pitch positioned between said 'catho'd e and plate to'exercise different rates" of "control at different portions of. the electron stream' between saidcathode and plate, the said screen grid includingat least twocylindrical sections of difi'erent diameters to exercise a decreased accelerating action upon the electron stream at those portions of the streamat which 'the control action of said control grid is 'de-' creased.

2. An electron discharge tube comprising a cylindrical unipotential cathode, a controlgrid sur-.

rounding said cathode and exerting different tron stream from said cathode, atleast onegridlike auxiliary cold electrode surrounding said- 'plate electrode surrounding said auxiliary electrode. I

3. An electron discharge tubecomprising a cathode, a helical control grid'surroun'ding said rates of control on difierent portions 'ofthe' elec cathode and having adjacentportions of different 7' pitches, a helical screen grid surrounding said control grid and comprising two sections each of the same pitch but of different diameters mounted end to end with the larger diameter section inreg'istry with the more openportion' of said plate electrode surrounding 4. An' electron discharge tube comprising a cathode, a control grid having agridwirewouncl into a helix with a pitch progressively increasing fromone end of said grid to the other, a helical screen grid of at least two sections each of uniform pitch, said sections being of diiferent diameters and mounted with the sections of smallest diameter surrounding the more closely wound end of said grid, and a cylindrical plate electrode, said grids, and said plate being mounted coaxially with and surrounding said cathode.

5. An electron discharge device comprising a cylindrical unipotential cathode, a control grid surrounding said cathode and comprising a helix of progressively increasing pitch, and another grid-like electrode surrounding said control grid and of difi'erent diameters at different points along its length, the portion of greatest diameter being in registry with the control grid portion of greatest pitch.

6. An electron discharge device comprising a cylindrical unipotential cathode, a control grid surrounding said cathode and comprising a helix of progressively increasing pitch, and another grid-like electrode surrounding said control grid and having at least two parts of difierent diameters, the part of lesser diameter being in registry with the control grid portion of lesser pitch and the part of greater diameter in registry with the control grid portion of greater pitch.

7. An electron discharge device comprising a cathode, an anode, a control grid having portions of different pitch, and a screen grid having two sections of uniform pitch and different diameters, with the section of greater diameter in registry with the portion of said control grid of greater pitch.

8. An electron discharge device comprising a cathode, an anode, a perforated control grid more open near one end than the other to exert at the more open end less control on the electron stream from the cathode to said anode, and a screen grid surrounding said control grid and having sections of different diameters mounted end to end, each of said sections being of uniform diameter throughout.

9. An electron discharge device comprising a cathode, an anode, a control grid of progressively increasing pitch, and a screen grid coextensive with said control grid and larger at one end than at the other.

10. An electron discharge tube comprising a cathode, a helical control grid surrounding said cathode and having portions of diflerent pitch at difierent points along its length, and another grid-like electrode surrounding said control grid and having two cylindrical portions of different diameters in registry with the portions of said control grid of difierent pitch.

1].. An electron discharge tube comprising a cathode, a control grid having at different points along its length different rates of control of the electron stream from said cathode, and a cooperating electrode having cylindrical sections of different diameters surrounding said control grid at difierent points along its length.

HAROLD A. SNOW. 

